When designing “realistic” dragons many people seem to default to chemical explosions in the mouth to produce streams of flame. While there is a precedent among animals, the bombardier beetle, vertebrates have more complex tissues, and a common issue that’s rarely addressed is how these animals don’t burn their mouths.
Here, I propose a solution that I’m surprised is not used more often, and with basis on medieval bestiary depictions of dragons. There, a common emphasis is how dragons shine or glow, sometimes even as a symbolism for the fall of Lucifer. Fire-breathing as a main dragon weapon is comparatively recent, but bioluminescence apparently is now.
Few land animals developed bioluminescence (most notably fireflies), but this seems to be more of a consequence of their habitats not really favouring it as well as complex tissues making light harder to pass rather than a deathly constraint, as evidence by lab mice with jellyfish-derived bioluminescence genes. Perhaps to make it slightly more plausible this hypothetical dragon (which is an enantiornithean bird for funzies) could have sacks full of light-emiting bacteria as anglerfish, but for my personal indulgence these dragons have produced their own luciferase and luciferin autogenically, perhaps as a consequence of powered flight.
The luminous patches are located throught the mouth and throat. Initially they likely developed for display or attracting insect prey in low light conditions, essentially a glowier version of the colourful mouths of many birds. But at some point a series of cornea-like lenses began to form adjacently in the back of the tongue; much like antlers, these are possibly “domesticated” tumours, specifically eye teratomas. Mobile, these structures can overlap one another, increasingly focusing the beams of light as they move and adjust until the beam strong enough to set things on fire, or potentially even energized enough to directly cause radiation burns.
Due to this light being focused on a single laser, there is potential for the animal to burn its mouth. Likewise, the series of steps (acquiring bioluminescence, then lenses, then muscles to control the lenses to focus beams, then enough fine control to make cleansing rays) is clear enough instead of the surely more complicated series of steps to acquire chemical firebreathing.